首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   16122篇
  免费   1397篇
  国内免费   1278篇
耳鼻咽喉   159篇
儿科学   188篇
妇产科学   141篇
基础医学   2148篇
口腔科学   255篇
临床医学   2122篇
内科学   2495篇
皮肤病学   118篇
神经病学   976篇
特种医学   560篇
外国民族医学   15篇
外科学   1631篇
综合类   2590篇
现状与发展   1篇
一般理论   2篇
预防医学   1131篇
眼科学   534篇
药学   1555篇
  22篇
中国医学   717篇
肿瘤学   1437篇
  2024年   44篇
  2023年   255篇
  2022年   592篇
  2021年   846篇
  2020年   607篇
  2019年   520篇
  2018年   553篇
  2017年   515篇
  2016年   477篇
  2015年   768篇
  2014年   832篇
  2013年   762篇
  2012年   1121篇
  2011年   1288篇
  2010年   736篇
  2009年   649篇
  2008年   895篇
  2007年   924篇
  2006年   837篇
  2005年   955篇
  2004年   554篇
  2003年   502篇
  2002年   448篇
  2001年   365篇
  2000年   426篇
  1999年   404篇
  1998年   288篇
  1997年   243篇
  1996年   183篇
  1995年   183篇
  1994年   158篇
  1993年   143篇
  1992年   129篇
  1991年   122篇
  1990年   91篇
  1989年   86篇
  1988年   69篇
  1987年   64篇
  1986年   51篇
  1985年   40篇
  1984年   15篇
  1983年   12篇
  1982年   9篇
  1981年   8篇
  1980年   3篇
  1979年   9篇
  1974年   2篇
  1973年   4篇
  1972年   2篇
  1960年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
BackgroundThere have been a few studies regarding the pre-attack symptoms (PAS) and pre-episode symptoms (PES) of cluster headache (CH), but none have been conducted in the Chinese population. The purpose of this study was to identify the prevalence and features of PAS and PES in Chinese patients, as well as to investigate their relationships with pertinent factors.MethodsThe study included patients who visited a tertiary headache center and nine other headache clinics between January 2019 and September 2021. A questionnaire was used to collect general data and information about PAS and PES.ResultsAmong the 327 patients who met the CH criteria (International Classification of Headache Disorders, 3rd edition), 269 (82.3%) patients experienced at least one PAS. The most common PAS were head and facial discomfort (74.4%). Multivariable logistic regression analysis depicted that the number of triggers (OR = 1.798, p = 0.001), and smoking history (OR = 2.067, p = 0.026) were correlated with increased odds of PAS. In total, 68 (20.8%) patients had PES. The most common symptoms were head and facial discomfort (23, 33.8%). Multivariable logistic regression analysis showed that the number of triggers were associated with increased odds of PES (OR = 1.372, p = 0.005).ConclusionsPAS are quite common in CH patients, demonstrating that CH attacks are not comprised of a pain phase alone; investigations of PAS and PES could help researchers better understand the pathophysiology of CH.  相似文献   
82.
83.
This study aimed to investigate the current status and influencing factors of kinesiophobia in patients after insertion of peripherally inserted central catheter (PICC).A total of 240 patients with PICC were included. Their postinsertion status and influencing factors were investigated using the general information questionnaire, Tampa Scale of Kinesiophobia (TSK), Medical Coping Modes Questionnaire, Numerical Rating Scale, and Self-rating Anxiety Scale.The mean TSK score was 36.49 ± 4.19 points, and 89 patients (37.08%) had kinesiophobia. Multiple linear regression analysis showed that factors such as education level, age, monthly income level, catheterization history, face, pain level, anxiety, and number of needle insertions influenced postoperative kinesiophobia in patients with PICC (P < .05). The total variation in the TSK score was 71.8%.The incidence of kinesiophobia was relatively high after PICC insertion. The medical staff needs to undertake targeted intervention measures to help minimize kinesiophobia after PICC insertion, allowing patients to perform scientifically correct functional exercises and attain physical recovery.  相似文献   
84.
The macromorphic properties of carbon nanotubes perform poorly because of their size limitations: nanosize in diameters and microsize in length. In this work, to realize these dual purposes, we first used an electrochemical method to tear the surface of multiwalled carbon nanotubes (MWCNTs) to anchor photonic Eu3+-complexes there. Through the polar reactive groups endowed by the tearing, the Eu3+-complexes coordinate at the defected structures, obtaining the Eu3+-complex-anchored, unzipped, multiwalled carbon nanotubes (E-uMWCNTs). The controllable surface-breaking retains the MWCNTs’ original, excellent mechanical properties. Then, to obtain the macromorphic structure with infinitely long fibers, a wet-spinning process was applied via the binding of a small quantity of polyvinyl alcohol (PVA). Thus, the wet-spun fibers with high contents of E-uMWCNTs (E-uMWCNT-Fs) were produced, in which the E-uMWCNTs took 33.3 wt%, a high ratio in E-uMWCNT-Fs. On the other hand, due to the reinforcing effect of E-uMWCNTs, the highest tensile strength can reach 228.2 MPa for E-uMWCNT-Fs. Meanwhile, the E-uMWCNT-Fs show high-efficiency photoluminescence and excellent media resistance performance due to the embedding effect of PVA on the E-uMWCNTs. Therefore, E-uMWCNT-Fs can exhibit excellent luminescence properties in aqueous solutions at pH 4~12 and in some high-concentration metal-ion solutions. Those distinguished performances promise outstanding innovations of this work.  相似文献   
85.
The excessive consumption of sugar-sweetened beverages (SSBs) has been proven to be critical for obesity among preschoolers. This study aimed to describe the SSB consumption rates among preschoolers in the Dongcheng District of Beijing, China, and to explore the association between obesogenic environmental determinants and consumption. We applied a stratified cluster sampling method and recruited 3057 primary caregivers of preschoolers in June 2019 to participate in the survey. The caregivers reported their children’s consumption rates of six categories of SSBs and their exposure rates to SSB-related obesogenic environments. The associations between them were tested using multivariate logistic regression models. The mean (SD) age of the children was 5.6 (0.6) years and nearly half (48.3%) were girls. About 84.5% of the children had consumed SSBs over the past three months, and sugar-sweetened milk beverages had the highest consumption rate. Higher exposure to advertisements for the corresponding SSB categories in children, higher frequency rates of consuming SSBs and of taking children to fast-food restaurants in caregivers, and lower frequency rates of reading the Nutrition Facts Panels by caregivers were associated with higher SSB consumption rates among children (p < 0.05 in all of the SSB categories investigated, except for the Nutrition Facts Panel reading behaviors for the sports and energy beverages). SSB consumption among preschoolers is of concern, and comprehensive policy actions and education are urgently needed.  相似文献   
86.
An N-addition reaction between imides and propargyl sulfonium salts was developed to afford sulfur-containing N-vinylimides with moderate to excellent yields. Under the activation of NaOAc·3H2O, imides could undergo deprotonation and propargyl sulfonium salts could isomerize to allenic sulfonium salts. The N-nucleophilic attack initiates the reaction and gives the desired products. Various imides, including arylimides, aliphatic imides and N-(arylsulfonyl) alkyl acylamides, and even bioactive saccharin, thalidomide and pomalidomide could provide organosulfur N-vinylimides compounds. The simple, mild and metal-free reaction conditions, the broad scope of substrates, gram-scale synthesis and convenient transformation embody the synthetic superiority of this process.

An N-addition reaction between imides and propargyl sulfonium salts was developed to afford sulfur-containing N-vinylimides with moderate to excellent yields.

N-vinylimides represent crucial structural motifs due to the importance of these frameworks (Fig. 1), which are the core structure found in biologically active structures,1 functional materials2 and natural products such as the parazoanthines A–E.3 They can also serve as versatile synthetic intermediates in the synthesis of β-2-amino acid derivatives4 and other complex structures.5Open in a separate windowFig. 1Representative functional N-vinylimides scaffolds.Due to the importance of these N-vinylimides frameworks, the growing interest in this featured moiety has catalyzed a recent spurt of attention for methodology appropriate for its construction. Conventionally, protocols for the synthesis of this important substrate class included transition-metal-catalyzed en-imidic C(sp2)–N bond formation reactions of imides with vinyl halides, pseudohalides or alkynes. The main strategies involved Cu-catalyzed Chan–Lam–Evans reactions,6 Ru-catalyzed hydroimidation reaction of imide with alkyne7 and Pd-catalyzed oxidative amination of alkenes.8 In 2021, Sandtorv''s group reported Cu-catalyzed Chan–Lam–Evans reaction for coupling cyclic imides and alkenylboronic acids by forming C(sp2)–N-bonds, enables the practical and mild preparation of (E)-enimides (Scheme 1a).6a In 2020, Schaub''s group reported Ru-phosphine catalyzed hydroimidation reaction of cyclic amides with acetylene under low pressure, affording new method for synthesis of N-vinylimides (Scheme 1b).7a Hull''s group reported Pd-catalyzed anti-Markovnikov oxidative amination reaction, alkenes are shown to react with imides in the presence of a palladate catalyst to generate the terminal imide, providing mild and robust complementary routes (Scheme 1c).8a Besides, rarely examples of organo-catalytic conjugate additions of imides to acetylene can also provide methods for the synthesis of N-vinylimides.9Open in a separate windowScheme 1Approaches for vinylation of imides.The previously reported synthesis strategies mainly involved the use of expensive Ru and Pd-catalysts, otherwise toxic copper catalyst, and the structural limitations imposed to phthalimide and therefore specialized. Considering the limitation in generality, the harsh reaction condition, and the use of metal-catalysis decreased the attractiveness for synthetic applications, the development of new kind of vinylation reagents and their application of building N-vinylimides in a simple, mild, metal-free and efficient manner are highly desirable.Our earlier work inspired our interest in synthesis of N-vinylimides by employing propargyl sulfonium salts as vinylation reagent. We have been exploring new reaction patterns of sulfonium salts and developed propargyl sulfonium salts involved [3 + 2] annulation/substitution reaction and N-addition/[2,3]-sigmatropic rearrangement reaction in an acyclic model.10 Based on our processive interests on constructing N-functionalized vinylation reaction and exploring the diverse reactive pathway of propargyl sulfonium salts, we herein report the realization of inorganic base promoted N-addition reaction of imides and propargyl sulfonium salts, delivering potential bioactive sulfur-containing N-vinylimides in moderate to excellent yields (Scheme 1).We began our investigation by selecting phthalimide 1a and propargyl sulfonium salt 2a as model substrates ().Optimization of the reaction conditionsa
EntryBaseSolventTemp. (°C)Yieldb (%)
1NaOAcCH3CN5045
2Na2CO3CH3CN5042
3K2CO3CH3CN5046
4Cs2CO3CH3CN5039
5KOHCH3CN5024
6LiOAc·2H2OCH3CN5033
7LiOAcCH3CN5042
8NaHCH3CN5040
9KOtBuCH3CN5037
10Et3NCH3CN5036
11DBUCH3CN5033
12NaOAc·3H2OTHF5037
13NaOAc·3H2OCHCl35034
14NaOAc·3H2ODCE5028
15NaOAc·3H2ODCM5035
16NaOAc·3H2OToluene50Trace
17cNaOAc·3H2OCH3CN2211
18cNaOAc·3H2OCH3CN3023
19cNaOAc·3H2OCH3CN6061
20cNaOAc·3H2OCH3CN8051
21cNaOAc·3H2OCH3CN9049
Open in a separate windowaUnless otherwise noted, the reactions were performed under air and imide 1a (0.3 mmol, 1.0 equiv.), base (0.45 mmol, 1.5 equiv.) in solvent (3.0 mL, c = 0.1 M) were mixed, the reaction mixture was stirred for 10 min at 22 °C. Then propargyl sulfonium salt 2a (0.45 mmol, 1.5 equiv.) was added in one portion. The reaction was stirred at 50 °C for 6 h until starting material 1a was fully consumed (monitored by TLC).bIsolated yield. DCE: 1,2-dichloroethane; DCM: dichloromethane.cWith the ratio of 1a : 2a : NaOAc·3H2O = 1 : 1.5 : 1.5.Having established the optimized conditions, we commenced to explore the substrate scope of the reaction. A selection of arylimides and aliphatic imides was next investigated with propargyl sulfonium salt 2a in Scheme 2. Generally, arylimides containing electron-withdrawing group such as tetrachloro-, 4-bromo-, 4-nitro- and 3-nitrophthalimide provided desired N-vinylimides products 3b–3e with moderate yields (Scheme 2, 3b–3e, with yields of 27–52%), probably due to the electron withdrawing effect of substituents. 1,8-Naphthalimide and 2,3-naphthalimide were well-tolerated to provide N-vinylimide products 3f and 3g with 62 and 65% yields, respectively. 3,4-Pyridinedicarboximide could also be engaged in the reaction to obtain 3h with yield of 43%. Subsequently, we went on to evaluate the reactivity of aliphatic imides. Unexpected, maleimide could not provide the desired N-nucleophilic addition product under the optimized conditions with recovering of the starting material. Oppositely, the method was high yielding and tolerable to succinimide and substituted succinimides. Succinimide and substituted succinimides worked well to deliver N-vinylimides products 3j–3o with moderate to excellent yields of 52–93%.11 Continuously, we evaluated the reactive effectiveness of glutarimide and substituted glutarimides. Under optimized conditions, glutarimide and substituted glutarimides could also react with 2a and give desired products 3p, 3q and 3r with yields of 32, 24 and 24%, respectively.Open in a separate windowScheme 2Scope of imidesa. aUnless otherwise noted, the reactions were performed under air and imide 1 (0.3 mmol, 1.0 equiv.), NaOAc·3H2O (0.45 mmol, 1.5 equiv.) in CH3CN (3.0 mL, c = 0.1 M) were mixed, the reaction mixture was stirred for 10 min at 22 °C. Then propargyl sulfonium salt 2a (0.45 mmol, 1.5 equiv.) was added in one portion. The reaction was stirred at 50 °C for 6 h until starting material 1 was fully consumed (monitored by TLC). bIsolated yield.Surprising reaction appeared when we explored the reaction of tetrahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione 1s and 1t (Scheme 3).12 Under the optimized conditions, 1s worked well with propargyl sulfonium salt 2a to deliver the corresponding product 3s with yield of 65%. Under the same conditions, compound 1t gave the desired N-vinylimide product 3t with yield of 35%, meanwhile with the unexpected N-vinylimide product 3i with yield of 16%, which unavailable in the reaction of maleimide with propargyl sulfonium salt 2a, probably due to the retro-Diels–Alder reaction of 1t with generation of maleimide intermediate.Open in a separate windowScheme 3Scope of imides.The reaction performance could also be adapted to N-(arylsulfonyl) alkyl acylamides (Scheme 4). The method smoothly transferred electron-deficient aryl sulfonyl acylamides to form N-vinylimide products such as 5a–5e in moderate yields. In contrast, when N-(arylsulfonyl) aryl acylamides 5i and N-(arylacyl) alkyl acylamides 5j–5l were involved, the reaction was sluggish and no desired N-vinylimide products could be obtained probably due to its low nucleophilicity (Scheme 4, 5i–5l).Open in a separate windowScheme 4Scope of aryl sulfonyl amides and carbonimidesa, aUnless otherwise noted, the reactions were performed under air and imide 4 (0.3 mmol, 1.0 equiv.), NaOAc·3H2O (0.45 mmol, 1.5 equiv.) in CH3CN (3.0 mL, c = 0.1 M) were mixed, the reaction mixture was stirred for 10 min at 22 °C. Then propargyl sulfonium salt 2a (0.45 mmol, 1.5 equiv.) was added in one portion. The reaction was stirred at 50 °C for 6 h until starting material 4 was fully consumed (monitored by TLC). bIsolated yield.To further broaden the scope of the reaction, other representative propargyl sulfonium salts were also investigated (Scheme 5). Trimethylsilyl contained propargyl sulfonium salt 2b could be applied to the reaction and the desilylation product 3a was obtained with a yield of 63%. The method was high yielding and tolerable to diverse bioactive molecules, such as saccharin, thalidomide and pomalidomide. Saccharin derivatives have been reported as good hCAs inhibitors,13 and thalidomide and pomalidomide belongs to an important class of molecules known as immunomodulatory imide drugs (IMiDs).14 We found that under optimized conditions, saccharin, thalidomide and pomalidomide were also compatible with propargyl sulfonium salt 2a and provided the corresponding products 7, 9 and 11 with 52, 87, and 80% yields, respectively (Scheme 5).Open in a separate windowScheme 5Scope of propargyl sulfonium salts and bioactive molecules.To demonstrate the synthetic utility of this protocol, we performed the gram-scale operation using phthalimide 1a (1.01 g, 6.8 mmol) and propargyl sulfonium salt 2a (1.5 equiv.) as the representative substrates under the optimized conditions, providing the related product 3a (1.03 g) with 65% yield (Scheme 6). The typical transformation was also conducted by oxidation of compound 3a with m-chloro peroxybenzoic acid (3.0 equiv.) and sulfonyl product 12 was obtained with 94% yield.Open in a separate windowScheme 6Gram-scale synthesis and further transformation.According to the previous reports on α-alkylidene pyrazolinones and propargyl sulfonium ylides,10b,c a possible mechanism is proposed to account for the formation of N-vinylimides 3 (Scheme 7). Under the activation of inorganic base NaOAc·3H2O, the imides 1 may undergo deprotonation to form intermediate I and propargyl sulfonium salt 2a can isomerize to allenic sulfonium salts II. The N-nucleophilic attack of I to allenic sulfonium salts II initiates the reaction and gives intermediate III. Subsequently, protonation of the species III and release of MeBr provided the desired product 3.Open in a separate windowScheme 7Plausible reaction mechanism.In summary, we have developed NaOAc·3H2O promoted N-addition reaction between imides and propargyl sulfonium salts, delivering potentially bioactive N-vinylimides in moderate to excellent yields. Various imides, including arylimides, aliphatic imides and N-(arylsulfonyl) alkyl acylamides, even bioactive saccharin, thalidomide and pomalidomide could tolerate and function to provide organosulfur N-vinylimides compounds. Gram-scale synthesis and convenient transformations are also furnished. The simple, mild, metal-free and efficient reaction condition, the broad scope of substrates, gram-scale synthesis and convenient transformation embody the synthetic superiority of this reaction process.  相似文献   
87.
目的调查某单位由腺病毒7型引发的呼吸道传染病疫情。方法用现场流行病学调查、临床检查和实验室检测等方法,对1起由腺病毒7型引发的呼吸道传染病疫情进行调查。结果 2013年1月13日~2013年2月4日,共有1400人的某单位突发呼吸道传染病疫情,其中440例(31.43%)出现急性呼吸道感染,主要症状有咳嗽(95.45%),381例有咽痛(86.59%),375例咳白痰(85.23%);全部有咽部充血;白细胞计数在(4.00~9.00)×109/L范围内有283例(64.32%);肝功异常36例(8.18%);肺炎支原体抗体阳性患者74例(16.82%);存在肺部CT异常266例(60.45%);腺病毒基因阳性111份(25.23%),其中82份(73.87%)标本的反应产物经测序证实为腺病毒7型。均经过抗感染及抗病毒治疗好转出院。结论此次疫情主要是由腺病毒7型引发的急性呼吸道感染,具有传播快、感染率高的特点,多为轻症呼吸道症状。  相似文献   
88.
89.
目的 探讨原发性肝癌的病理解剖学特征及其意义.方法 2000年8月至2007年12月共有89例肝硬化合并原发性肝癌患者接受肝移植手术,收集手术切除标本及受侵犯的邻近组织和肿大的淋巴结.将标本沿冠状切面每隔1.0 cm切开,观察肝癌的大小、数目、分布、包膜、门静脉瘤栓、卫星灶,并记录卫星灶与主瘤问的距离、受肝癌侵犯的组织、淋巴结等,然后送病理学检查,最终诊断以病理报告为准.结果 89例患者中,肝细胞肝癌86例,胆管细胞性肝癌3例;多发肿瘤53例,单发肿瘤36例;13例主瘤在右叶,但左叶存在小癌灶;14例有较完整的包膜,75例无包膜,无包膜病例中11例边界不清;术中所见侵犯邻近组织的25例患者中,8例经病理证实,16例的肿大淋巴结中7例有癌浸润;有门静脉主干或分支瘤栓者42例(47.2%),有卫星灶者39例(43.8%),瘤栓与卫星灶随肿瘤体积、数目增加而增加;卫星灶距主瘤距离多为0.5~3.0 cm,少数达5.0 cm.结论 全肝切除标本能全面地反映肝癌牛长和浸润特征.应警惕门静脉瘤栓及卫星灶的存在,特别是距主瘤较远的卫星灶及门静脉小支内的瘤栓,以及分布在另一肝叶的小癌灶.这些因素在影像学检查时容易漏诊,但会影响肝切除术或肝移植术的疗效.  相似文献   
90.
乌司他丁对体外循环术中肺脏的保护作用   总被引:3,自引:0,他引:3  
目的研究乌司他丁对体外循环术中肺脏的保护作用。方法选择心脏直视手术患者60例,分别设乌司他丁治疗组和对照组各30例,在5个时间点分别检测中性粒细胞弹性酶(NE)、白介素(IL)-8、超氧化物歧化酶(SOD)含量的变化。结果在主动脉开放后的3个时间点两组数据具有显著性差异。结论乌司他丁在体外循环中具有抗蛋白酶分解、调节细胞因子和减少氧自由基的肺保护作用。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号